The sampling of video signals emanating from an analog source is well known. It uses the Shannon-Nyquist theorem. According to this theorem, if the passband of a signal is limited to a frequency range such as [0,Fmax], it is necessary and sufficient to sample this signal at a minimum frequency 2.times.Fmax in order to be capable of reconstructing it from these samples. This constraint is manifested by the introduction of low-pass filters whose purpose is to limit the spectrum of the signals before sampling. In this case, the phase of the clock signal is of no importance in the sampling procedure. Indeed, the same signal sampled by two clocks of the same frequency but which are out of phase contains the same information to within a constant lag.
The same does not hold when having to sample video signals emanating from a computer device, namely, signals of digital origin. Indeed, the spectrum of these signals is very wide and they are intended to be viewed under the highest possible resolution. Accordingly, the passband must not be limited since there would be a loss of fineness. If signals of this type have to be injected into a device which comprises a sampling stage, the following problems arise:
If the inbound signal is filtered so as to limit its passband and satisfy the Nyquist criteria, the response of the filter to digital-type signals exhibiting steep transitions will engender overoscillations which are very prejudicial to the sharpness of the characters. PA1 If the inbound signal is hardly filtered so as to avoid overoscillation, the attenuation afforded to the frequency components will be insufficient to avoid a likewise prejudicial spectral aliasing. PA1 If the inbound signal is sampled without prior filtering, it is imperative to adopt not only the exact frequency which served to generate the signal but also a sampling phase corresponding to the middle of each porch. PA1 gauging of the position of the edges of the computer-type signals with respect to the sampling clock with the aid of an analog ramp triggered by the rising edges of the computer-type signals in such a way as to obtain a first value dependent on the said position, PA1 carrying out of a sampling clock phase correction, PA1 carrying out of a frequency correction,
The problem is all the more complex since there is no predefined fixed standard in this area. Indeed, in the displaying of video signals emanating from a graphics card, only the number of active pixels per line of the source and the number of active lines per image of the source are defined. Accordingly, the total number of pixels per line, the total number of lines as well as the image frequency and the pixel frequency are not standardized. Similarly, the phase of the first active pixel with respect to the edge of the synchronizing clock is not defined, neither linewise nor imagewise.